Tiny water flea has the biggest genome ever sequenced

The water flea species Daphnia pulex is barely more than a millimeter long, is completely translucent, has no clear divisions between their various body parts...and has the largest genome of any animal we've ever encountered. It has an incredible 31,000 genes in its genome, over 8,000 more than humans have. So what's such a tiny organism doing with such a huge amount of genes?

The most basic reason is that Daphnia pulex is involved in runaway gene duplication, as it constantly churns out copies of genes at a far greater rate than other organisms. Project leader John Colbourne explains:

"Daphnia's high gene number is largely because its genes are multiplying, by creating copies at a higher rate than other species. We estimate a rate that is three times greater than those of other invertebrates and 30 percent greater than that of humans."

Daphnia organisms tend to hang onto their genes once they've got them, as most members of the species prefer asexual reproduction unless extreme ecological conditions make the stronger offspring created by sexual reproduction a necessity.

Researcher Don Gilbert points out that at least a third of the Daphnia genome is made up of completely unknown genes that have never before been documented. Unknown genes aren't in themselves all that shocking - even incredibly well-researched genomes like that of the fruit fly Drosophila can still hold some surprises - but, again, the sheer number of unknown genes is pretty staggering.

A lot of Daphnia's genes appear to have clear environmental functions, as certain genes are expressed in response to changing environmental factors. For instance, if one of these water fleas finds itself in polluted waters, a whole suite of genes will be expressed slightly differently than in its counterparts that are still in clean water. These shifting ecological reactions seem to be tied to Daphnia's frequent cell duplication, although the researchers say we still don't entirely understand this relationship.

Beyond what it can tell us about the extremes of genomics and gene expression, Daphnia might have some more practical uses. Its ability to respond immediately to different ecological conditions, including the presence of pollutants, could provide us with a much faster and cheaper way of checking the quality of our water supplies than other currently available tests.

Via Science. Image by Dr. Jan Michels/Nikon Small World.